Yearly flooding along the lower Nile (green swath, above) was critically important for agriculture in ancient Egypt, but a new study suggests climate-changing volcanic eruptions occasionally stifled floods.

Famine is no pharaoh’s friend—just ask Cleopatra or Ptolemy III. But those rulers may have had more to blame than just bad luck: According to a new study, volcanic eruptions around the ancient world likely suppressed the Nile’s annual floods—critical for agriculture—by altering rainfall upriver in the Ethiopian highlands several times from the third to first centuries B.C.E. The climatic consequences of those eruptions may have helped trigger tax riots and other forms of social unrest, social scientists say.

“This is a terrific combination of scientific and humanities research,” says Graham Oliver, a historian at Brown University who wasn’t involved in the new analysis. “[It’s] a really important contribution to our understanding of the ancient world.”

Long before the use of sophisticated irrigation equipment, those along the lower Nile relied on natural flooding in the late summer and fall to deliver water—and fertile sediment—to the floodplains where they farmed their crops, says Francis Ludlow, a historical climatologist at Trinity College in Dublin. Without the floods, the soil became parched, boosting the risk of crop failure the following year. On average, 85% of the Nile’s flow originates from rains in the highlands of Ethiopia.

Elsewhere in the world, volcanoes were erupting, as shown by higher levels of sulfates in ice cores from Greenland and Antarctica. In modern times, such eruptions have been linked to low summer flooding along the Nile. To find out just how low, Ludlow and his colleagues turned to the Islamic Nilometer, the longest running annual measurement of river flow, which records the highest level of the Nile from 622 to 1902. The team found that flood levels in the 60 years with volcanic eruptions were about 22 centimeters lower, on average, than they were in years without eruptions, they report today in Nature Communications.

Once the team discovered the link between volcanic eruptions and reduced Nile flooding, they turned their attention to more ancient times—specifically, to the period from 305 B.C.E. to 30 B.C.E., when Greeks ruled a dynasty centered in Egypt. They used climate simulations to estimate the effects of volcanic eruptions on summer monsoon rainfall in the Ethiopian highlands. Those analyses suggested that large eruptions indeed reduced the rainfall needed to trigger fall floods along the lower Nile. This likely happened when volcano-spewed sulfur dioxide wafted high into the atmosphere, scattering sunlight back into space and briefly lowering global temperature and changing precipitation patterns, among other effects.

“It’s clear that volcanic eruptions systematically suppressed Nile flow,” says Joseph Manning, a historian at Yale University and co-author of the new analysis.

Separate analyses of documents from ancient Egypt—everything from inscriptions on monuments to tax records, poems, and letters—hint that eruptions may have contributed to social unrest, including riots, tensions between Egyptians and their Greek overlords, famines and plagues, and farmers abandoning their land and moving to the cities. In some cases, these effects came soon after the eruptions; in other cases, they didn’t occur until the following year. Ludlow and his colleagues are quick to point out that volcanic eruptions didn’t directly cause social unrest, but were instead possible triggers for already simmering tensions. “Many people think that history unfolds on a blank chessboard and that the environment is not a factor, but of course it is,” Ludlow says.

The team’s analysis is very convincing, says Heli Huhtamaa, a climate historian at Utrecht University in the Netherlands. Using a variety of records “is an important part of putting together the effects of the eruptions,” she notes.

But some scientists are skeptical, including Kenneth Verosub, a geophysicist at the University of California, Davis. For one thing, he thinks the team should have compared the results of their climate simulations with real-world data such as tree-ring data from the Ethiopian highlands.

Interpreting ancient documents is “often very complicated,” Manning says. Plus, he notes, for some periods within the 3 centuries of Greek rule in Egypt, documents such as tax records aren’t readily available. But that lack may itself be a clue, he says, suggesting a loss of state control of Egyptian society.